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| 1 | +--- |
| 2 | +title: Spring Session, JSESSIONID, and Server Detection |
| 3 | +lang: en |
| 4 | +layout: post |
| 5 | +audio: false |
| 6 | +translated: false |
| 7 | +generated: true |
| 8 | +image: false |
| 9 | +--- |
| 10 | + |
| 11 | +Let’s break this into two parts: how **Spring Session/JSESSIONID** works, and how to tell whether Spring is running on **Tomcat or Netty**. |
| 12 | + |
| 13 | +--- |
| 14 | + |
| 15 | +## 1. How Spring Session and `JSESSIONID` Work |
| 16 | + |
| 17 | +* **JSESSIONID** is the standard HTTP session identifier used by servlet containers (Tomcat, Jetty, Undertow, etc.). |
| 18 | +* When a client makes the first request that requires a session, the container (or Spring Session if you use it) generates a random session ID, stores session data on the server, and sends the ID back to the client in a `Set-Cookie` header. |
| 19 | +* The client includes this `JSESSIONID` cookie on subsequent requests, allowing the server to find the correct session. |
| 20 | +* By default, the session data is stored **in the servlet container’s memory**. For example: |
| 21 | + |
| 22 | + * Tomcat stores sessions in memory (unless you configure persistence). |
| 23 | + * If you use **Spring Session**, session storage can be externalized (Redis, JDBC, Hazelcast, etc.), while still using the `JSESSIONID` cookie to identify users. |
| 24 | +* Spring Security and other Spring features rely on this session mechanism for authentication and state management. |
| 25 | + |
| 26 | +--- |
| 27 | + |
| 28 | +## 2. How to Know Whether Spring Uses Tomcat or Netty |
| 29 | + |
| 30 | +Spring Boot can run with different embedded servers: |
| 31 | + |
| 32 | +* **Tomcat** (default for Spring Boot starter-web) |
| 33 | +* **Jetty** (if you swap dependencies) |
| 34 | +* **Undertow** (alternative servlet container) |
| 35 | +* **Netty** (used by Spring WebFlux, not servlet-based) |
| 36 | + |
| 37 | +### Ways to Check: |
| 38 | + |
| 39 | +1. **Look at your dependencies** |
| 40 | + |
| 41 | + * If your `pom.xml` or `build.gradle` includes `spring-boot-starter-web`, it uses **Tomcat** by default. |
| 42 | + * If it includes `spring-boot-starter-webflux`, it uses **Netty** by default. |
| 43 | + |
| 44 | +2. **Check startup logs** |
| 45 | + |
| 46 | + * When the app starts, Spring Boot logs the embedded server. |
| 47 | + * Example (Tomcat): |
| 48 | + |
| 49 | + ``` |
| 50 | + Tomcat started on port(s): 8080 (http) |
| 51 | + ``` |
| 52 | + * Example (Netty): |
| 53 | +
|
| 54 | + ``` |
| 55 | + Netty started on port(s): 8080 |
| 56 | + ``` |
| 57 | +
|
| 58 | +3. **Check runtime classes** |
| 59 | +
|
| 60 | + * In code or via actuator, check what server classes are loaded. |
| 61 | + * Tomcat classes start with `org.apache.catalina.` |
| 62 | + * Netty classes start with `io.netty.` |
| 63 | +
|
| 64 | +--- |
| 65 | +
|
| 66 | +✅ **Summary**: |
| 67 | +
|
| 68 | +* `JSESSIONID` is just the cookie pointing to a server-side session, managed by Tomcat (default) or by Spring Session (Redis/DB). |
| 69 | +* You can tell whether Spring uses **Tomcat or Netty** by checking dependencies, startup logs, or runtime classes. |
| 70 | +
|
| 71 | +--- |
| 72 | +
|
| 73 | +Do you want me to also explain **how session handling differs between Spring MVC (Tomcat) and Spring WebFlux (Netty)**? That’s where the distinction matters most. |
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